1. Field of the Invention
This invention relates generally to vacuum sputter coating apparatus, and more particularly to planar magnetron sputtering source designs.
2. Description of Related Art
Cathodic sputtering refers to the erosion of a cathode by ion bombardment that occurs when an electrical discharge is passed between electrodes in a low pressure gas. In the sputtering process inert gas ions with a positive charge are accelerated from the glow discharge, that forms between the electrodes, to the negative cathode. Erosion results from the ejection of atoms and clusters of atoms from the cathode surface as a result of momentum transfer from the bombarding ions. Some of the ejected cathode material condenses on surfaces surrounding the cathode. Sputtering becomes a coating process when the ejected material is deliberately condensed on a substrate suitably positioned near the cathode.
Sputtering is a vacuum coating process where an electrically isolated cathode is mounted in a chamber that can be evacuated and partially filled with an inert gas. If the cathode material is an electrical conductor, a direct-current high-voltage power supply is used to apply the high voltage potential. If the cathode is an electrical insulator, the polarity of the electrodes must be reversed at very high frequencies to prevent the formation of a positive charge on the cathode that would stop the ion bombardment process. Since the electrode polarity is reversed at a radio frequency of 13.56 MHz, this process is referred to as RF-sputtering.
Magnetron sputtering is a more efficient form of diode sputtering that uses a magnetic field to trap electrons in a region near the target surface, thereby creating a higher probability of ionizing a gas atom. The high density of ions created near the target surface causes material to be removed many times faster than in diode sputtering. The magnetron effect is created by an array of permanent magnets included within the cathode assembly that produce a magnetic field normal to the electric field.
Ion bombardment not only causes atoms of the target material to be ejected, but it also imparts considerable thermal energy to the target. Consequently, any target attachment scheme must provide for good physical contact to the cathode assembly to allow adequate thermal transfer of the target's heat to the cooling media or away from the sputtering source. This is particularly true in the case of magnetron sputtering where very large ion currents are produced causing a very intense and localized heating of the target.
Various means have been used in the past for holding sputter targets in place within the sputter sources. Commercially available sputter coating target cathodes today are either bonded directly to the cathode assembly or secured using various mechanical means. The method used to attach the sputter target to the cathode assembly will also greatly affect the size and overall design of the magnetron sputtering source, the amount of down time when changing targets, and the overall performance of the source. By also eliminating the need for a cooling fluid, the miniaturization of the magnetron sputtering apparatus becomes feasible, and the cost of building the sputtering apparatus becomes more economical.
A co-pending application titled "Magnetically Attached Sputter Targets," by Makowiecki, et al., Ser. No. 07/962,657, filed Oct. 19, 1992, (now U.S. Pat. No. 5,286,361) discloses a novel method and assembly for attaching sputtering targets to cathode assemblies using a magnetically permeable material imbedded in the base portion of the sputter target. Target attachment to the cathode is achieved by virtue of the permanent magnets and/or pole pieces that create magnetic flux lines adjacent to the backing plate, which strongly attract the magnetically permeable material in the target assembly. The magnetically attached sputter target, in conjunction with the present invention, results in a magnetron sputtering source design that is compact, versatile, and less costly to manufacture and operate than prior art sputtering source designs.
The present invention also features the use of a thermally conductive and electrically insulating washer, which allows the mounting surface to be used as a heat sink, thereby removing the need for supplying (or reducing the amount of) cooling fluid to the sputtering source. This eliminates (or decreases the number of) cooling channels in the cathode assembly, thereby resulting in a more compact sputtering source design.
Further, the present invention also features a design that allows for a miniature and compact magnetron sputtering source. Greater flexibility is obtained when several small sources are used in an array instead of a larger single source. Precise control of the sputter coating thickness can be extended to virtually any size or shape surface by simply reconfiguring the arrangement of small sources in the array. In addition to process control and flexibility, the concept of using several small sources in an array also offers substantial cost savings in both fabrication and operation.
Furthermore, the present invention also features a small source design that does not require vacuum sealing. Eliminating the vacuum seal between the source and the sputtering chamber reduces the number of potential leak sites in the system.
U.S. Pat. No. 4,204,936 discloses a method and apparatus for attaching a target to the cathode of a sputtering system using a ferromagnetic retainer that is not incorporated in the sputter target assembly. Consequently, the retainer must be positioned on the sputtering surface of the target to allow it to clamp the target against the backing plate of the cathode assembly. The location of the retainer is in close proximity to the erosion area of the target. This degrades the operating characteristics of the sputtering apparatus by potentially allowing the retainer material to be sputtered away with the target material, thereby contaminating the substrate being coated. The patent also discloses a sputtering apparatus requiring the use of water passageways to remove heat from the cathode assembly. Heat transfer is further assisted by the use of thermally conductive liquid metals between the target and the backing plate.
U.S. Pat. No. 4,392,939 teaches a magnetron sputtering cathode design where the target is held against the backing plate by a vacuum, enabling the backing plate to be reused. The design is complicated in that close tolerances must be maintained to insure a strong vacuum seal between the target and the backing plate. The backing plate is water cooled and heat transfer is further assisted by the use of a molten solder between the target and the backing plate which adheres to the backing plate but not to the target. The design is not versatile, the vacuum channels are costly to machine into the backing plate, and the addition of a vacuum system increase the overall cost of building the apparatus. The inefficiency of the design is realized by the fact that the many channels required for achieving a good vacuum result in reducing the interfacial contact between the target and the backing plate. The design tries to compensate for this by using a selectively adhering molten metal to regain the thermal contact lost by the channels.
U.S. Pat. No. 4,417,968 discloses a cylindrical magnetron sputtering apparatus design for coating a large number of substrates. The design is very complex, bulky, costly to fabricate and to maintain. Heat transfer from the cathode assembly is facilitated by a set of water channels located inside the cylindrical cathode.
U.S. Pat. No. 4,421,628 shows a rectangular target design for cathode sputtering apparatus. The target is held in place by screws, located around the perimeter and in a groove centrally located in the face of the target, that thread into the backing plate. The simple mechanical design makes changing targets easy; however, the fact that screws are located at the sputtering surface near the erosion area offers a potential source for contamination.
U.S. Pat. No. 4,761,218 discloses a sputtering source with plural target rings that are individually controlled to adjust the erosion regions of the targets for coating large substrates and/or substrates of different shapes. The system requires that cooling fluid be injected into the cathode assemblies to carry heat away from the sources. Although the design does offer some versatility, its use is primarily for coating large substrates. The design is inherently complex and costly as each target ring requires a separate power supply and controller.
U.S. Pat. Nos. 4,049,533; 4,183,797; and 4,812,217 disclose ion sputtering devices for producing coatings or films. The devices comprise a plurality of vacuum chambers and pairs of target cathodes arranged opposite each other. The designs are complicated and considerable assembly and disassembly time are required to change targets. The designs are not versatile and their manufacture is costly.
U.S. Pat. No. 4,915,805 shows a hollow cathode magnetron sputtering apparatus for simultaneously sputter coating a substrate on both sides using two planar magnetrons. Cooling is accomplished by coolant tubes interspersed between the magnets and the target material in the cathode assembly. The design is complex and the cooling assembly is costly to fabricate. Further, the complexity of the design makes miniaturization difficult.
U.S. Pat. No. 5,009,765 teaches a sputter target design where the sputter target is welded to the backing plate. The backing plate is machined to provide for a bayonet mount to the corresponding cathode assembly. With this design, both the target and the backing plate materials must be discarded at the end of the target's life. The welded target plus backing plate assembly has a large cavity directly behind the target that acts as a reservoir for the cooling media to flow through to carry heat away from the assembly. The design is more costly as the backing plate must be discarded with the expended target. The requirement of a large cooling media cavity for heat transfer makes it difficult to use this target assembly in a compact sputtering apparatus design.
U.S. Pat. No. 5,021,139 teaches a cathode sputtering apparatus design with an improved cooling method that uses expandable cooling channels. The design uses a rectangular coolant channel that presses against the target and the backing plate to improve the conduction of heat away from the source. The size of the cooling channel is large relative to the size of the cathode assembly making miniaturization difficult. The cathode design is simple; however, the coolant channel itself requires a plurality of beads or folds that extend along the channel to allow the channel to change dimensionally under coolant pressure. The fabrication of the channels, consequently, adds considerable cost to the design.
U.S. Pat. No. 5,022,978 shows a sputtering apparatus for coating three dimensional articles where the article is rotated in the presence of a rectangular magnetron cathode. The target and the backing plate appear to be brazed together with coolant channels running through the backing plate-permanent magnet assembly. The joining of the target to the backing plate (containing the permanent magnets) causes the whole subassembly to be expended after the target expires. This adds considerable cost to the replacement of the target.
Several other designs that employ the circulation of cooling media directly through the backing plate located adjacent to the sputter target have been conceived. In some cases, the thermal contact between the target and the cooling wall are maintained by the thermal expansion of the target against the cooling wall. Examples of such sources are shown in U.S. Pat. Nos. 4,100,055; 4,385,979; 4,457,825; and 4,657,654. The use of a material's thermal expansion alone, however, is not an efficient way to maintain good thermal contact between the target and the cathode assembly. Because expansion is a function of temperature, differences in contact pressure are realized at different operating temperatures. This reduces the versatility of such designs and can introduce substantial degradation in the systems ability to efficiently carry heat away from the target under different operating conditions.
From the prior art it is recognized that all magnetron sputtering sources have four basic design features. First, they all contain a directly cooled and electrically isolated cathode assembly that supports the sputter target and contains the permanent magnet assembly.
Second, a ceramic or polymer insulator electrically isolates the cathode assembly from ground. The insulator structure also establishes the vacuum compatibility of the source by way of ceramic-to-metal braze joints or elastomer O-ring seals.
Third, only a portion of the power in the sputtering process is consumed by ejecting near surface atoms by ion bombardment. Most of the power contributes to thermal heating of the sputter target and the cathode assembly. The excess heat generated is removed by circulating water or another cooling media in the cathode assembly.
Fourth, the sputtering process should be confined to the target surface by using sputter shields that prevent all other exposed surfaces of the cathode assembly and/or target attachment hardware from also being sputtered. This is a necessary requirement if contamination is to be avoided.